package overload;
our $VERSION = '1.26';
%ops = (
with_assign => "+ - * / % ** << >> x .",
assign => "+= -= *= /= %= **= <<= >>= x= .=",
num_comparison => "< <= > >= == !=",
'3way_comparison' => "<=> cmp",
str_comparison => "lt le gt ge eq ne",
binary => '& &= | |= ^ ^= &. &.= |. |.= ^. ^.=',
unary => "neg ! ~ ~.",
mutators => '++ --',
func => "atan2 cos sin exp abs log sqrt int",
conversion => 'bool "" 0+ qr',
iterators => '<>',
filetest => "-X",
dereferencing => '${} @{} %{} &{} *{}',
matching => '~~',
special => 'nomethod fallback =',
);
my %ops_seen;
for $category (keys %ops) {
$ops_seen{$_}++ for (split /\s+/, $ops{$category});
}
sub nil {}
sub OVERLOAD {
$package = shift;
my %arg = @_;
my $sub;
*{$package . "::(("} = \&nil; # Make it findable via fetchmethod.
for (keys %arg) {
if ($_ eq 'fallback') {
for my $sym (*{$package . "::()"}) {
*$sym = \&nil; # Make it findable via fetchmethod.
$$sym = $arg{$_};
}
} else {
warnings::warnif("overload arg '$_' is invalid")
unless $ops_seen{$_};
$sub = $arg{$_};
if (not ref $sub) {
$ {$package . "::(" . $_} = $sub;
$sub = \&nil;
}
#print STDERR "Setting '$ {'package'}::\cO$_' to \\&'$sub'.\n";
*{$package . "::(" . $_} = \&{ $sub };
}
}
}
sub import {
$package = (caller())[0];
# *{$package . "::OVERLOAD"} = \&OVERLOAD;
shift;
$package->overload::OVERLOAD(@_);
}
sub unimport {
$package = (caller())[0];
shift;
*{$package . "::(("} = \&nil;
for (@_) {
warnings::warnif("overload arg '$_' is invalid")
unless $ops_seen{$_};
delete $ {$package . "::"}{$_ eq 'fallback' ? '()' : "(" .$_};
}
}
sub Overloaded {
my $package = shift;
$package = ref $package if ref $package;
mycan ($package, '()') || mycan ($package, '((');
}
sub ov_method {
my $globref = shift;
return undef unless $globref;
my $sub = \&{*$globref};
no overloading;
return $sub if $sub != \&nil;
return shift->can($ {*$globref});
}
sub OverloadedStringify {
my $package = shift;
$package = ref $package if ref $package;
#$package->can('(""')
ov_method mycan($package, '(""'), $package
or ov_method mycan($package, '(0+'), $package
or ov_method mycan($package, '(bool'), $package
or ov_method mycan($package, '(nomethod'), $package;
}
sub Method {
my $package = shift;
if(ref $package) {
local $@;
local $!;
require Scalar::Util;
$package = Scalar::Util::blessed($package);
return undef if !defined $package;
}
#my $meth = $package->can('(' . shift);
ov_method mycan($package, '(' . shift), $package;
#return $meth if $meth ne \&nil;
#return $ {*{$meth}};
}
sub AddrRef {
no overloading;
"$_[0]";
}
*StrVal = *AddrRef;
sub mycan { # Real can would leave stubs.
my ($package, $meth) = @_;
local $@;
local $!;
require mro;
my $mro = mro::get_linear_isa($package);
foreach my $p (@$mro) {
my $fqmeth = $p . q{::} . $meth;
return \*{$fqmeth} if defined &{$fqmeth};
}
return undef;
}
%constants = (
'integer' => 0x1000, # HINT_NEW_INTEGER
'float' => 0x2000, # HINT_NEW_FLOAT
'binary' => 0x4000, # HINT_NEW_BINARY
'q' => 0x8000, # HINT_NEW_STRING
'qr' => 0x10000, # HINT_NEW_RE
);
use warnings::register;
sub constant {
# Arguments: what, sub
while (@_) {
if (@_ == 1) {
warnings::warnif ("Odd number of arguments for overload::constant");
last;
}
elsif (!exists $constants {$_ [0]}) {
warnings::warnif ("'$_[0]' is not an overloadable type");
}
elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) {
# Can't use C above as code references can be
# blessed, and C would return the package the ref is blessed into.
if (warnings::enabled) {
$_ [1] = "undef" unless defined $_ [1];
warnings::warn ("'$_[1]' is not a code reference");
}
}
else {
$^H{$_[0]} = $_[1];
$^H |= $constants{$_[0]};
}
shift, shift;
}
}
sub remove_constant {
# Arguments: what, sub
while (@_) {
delete $^H{$_[0]};
$^H &= ~ $constants{$_[0]};
shift, shift;
}
}
1;
__END__
=head1 NAME
overload - Package for overloading Perl operations
=head1 SYNOPSIS
package SomeThing;
use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
...
package main;
$a = SomeThing->new( 57 );
$b = 5 + $a;
...
if (overload::Overloaded $b) {...}
...
$strval = overload::StrVal $b;
=head1 DESCRIPTION
This pragma allows overloading of Perl's operators for a class.
To overload built-in functions, see L instead.
=head2 Fundamentals
=head3 Declaration
Arguments of the C directive are (key, value) pairs.
For the full set of legal keys, see L below.
Operator implementations (the values) can be subroutines,
references to subroutines, or anonymous subroutines
- in other words, anything legal inside a C call.
Values specified as strings are interpreted as method names.
Thus
package Number;
use overload
"-" => "minus",
"*=" => \&muas,
'""' => sub { ...; };
declares that subtraction is to be implemented by method C
in the class C (or one of its base classes),
and that the function C is to be used for the
assignment form of multiplication, C.
It also defines an anonymous subroutine to implement stringification:
this is called whenever an object blessed into the package C
is used in a string context (this subroutine might, for example,
return the number as a Roman numeral).
=head3 Calling Conventions and Magic Autogeneration
The following sample implementation of C (which assumes
that C objects are simply blessed references to scalars)
illustrates the calling conventions:
package Number;
sub minus {
my ($self, $other, $swap) = @_;
my $result = $$self - $other; # *
$result = -$result if $swap;
ref $result ? $result : bless \$result;
}
# * may recurse once - see table below
Three arguments are passed to all subroutines specified in the
C directive (with exceptions - see below, particularly
L).
The first of these is the operand providing the overloaded
operator implementation -
in this case, the object whose C method is being called.
The second argument is the other operand, or C in the
case of a unary operator.
The third argument is set to TRUE if (and only if) the two
operands have been swapped. Perl may do this to ensure that the
first argument (C) is an object implementing the overloaded
operation, in line with general object calling conventions.
For example, if C and C are Cs:
operation | generates a call to
============|======================
$x - $y | minus($x, $y, '')
$x - 7 | minus($x, 7, '')
7 - $x | minus($x, 7, 1)
Perl may also use C to implement other operators which
have not been specified in the C directive,
according to the rules for L described later.
For example, the C above declared no subroutine
for any of the operators C, C (the overload key for
unary minus), or C. Thus
operation | generates a call to
============|======================
-$x | minus($x, 0, 1)
$x-- | minus($x, 1, undef)
$x -= 3 | minus($x, 3, undef)
Note the Cs:
where autogeneration results in the method for a standard
operator which does not change either of its operands, such
as C, being used to implement an operator which changes
the operand ("mutators": here, C and C),
Perl passes undef as the third argument.
This still evaluates as FALSE, consistent with the fact that
the operands have not been swapped, but gives the subroutine
a chance to alter its behaviour in these cases.
In all the above examples, C is required
only to return the result of the subtraction:
Perl takes care of the assignment to $x.
In fact, such methods should I modify their operands,
even if C is passed as the third argument
(see L).
The same is not true of implementations of C and C:
these are expected to modify their operand.
An appropriate implementation of C might look like
use overload '--' => "decr",
# ...
sub decr { --${$_[0]}; }
If the experimental "bitwise" feature is enabled (see L), a fifth
TRUE argument is passed to subroutines handling C, C, C and C.
This indicates that the caller is expecting numeric behaviour. The fourth
argument will be C, as that position (C) is reserved for use
by L.
=head3 Mathemagic, Mutators, and Copy Constructors
The term 'mathemagic' describes the overloaded implementation
of mathematical operators.
Mathemagical operations raise an issue.
Consider the code:
$a = $b;
--$a;
If C and C are scalars then after these statements
$a == $b - 1
An object, however, is a reference to blessed data, so if
C and C are objects then the assignment C
copies only the reference, leaving C and C referring
to the same object data.
One might therefore expect the operation C to decrement
C as well as C.
However, this would not be consistent with how we expect the
mathematical operators to work.
Perl resolves this dilemma by transparently calling a copy
constructor before calling a method defined to implement
a mutator (C, C, and so on.).
In the above example, when Perl reaches the decrement
statement, it makes a copy of the object data in C and
assigns to C a reference to the copied data.
Only then does it call C, which alters the copied
data, leaving C unchanged.
Thus the object metaphor is preserved as far as possible,
while mathemagical operations still work according to the
arithmetic metaphor.
Note: the preceding paragraph describes what happens when
Perl autogenerates the copy constructor for an object based
on a scalar.
For other cases, see L.
=head2 Overloadable Operations
The complete list of keys that can be specified in the C
directive are given, separated by spaces, in the values of the
hash C:
with_assign => '+ - * / % ** << >> x .',
assign => '+= -= *= /= %= **= <<= >>= x= .=',
num_comparison => '< <= > >= == !=',
'3way_comparison'=> '<=> cmp',
str_comparison => 'lt le gt ge eq ne',
binary => '& &= | |= ^ ^= &. &.= |. |.= ^. ^.=',
unary => 'neg ! ~ ~.',
mutators => '++ --',
func => 'atan2 cos sin exp abs log sqrt int',
conversion => 'bool "" 0+ qr',
iterators => '<>',
filetest => '-X',
dereferencing => '${} @{} %{} &{} *{}',
matching => '~~',
special => 'nomethod fallback ='
Most of the overloadable operators map one-to-one to these keys.
Exceptions, including additional overloadable operations not
apparent from this hash, are included in the notes which follow.
This list is subject to growth over time.
A warning is issued if an attempt is made to register an operator not found
above.
=over 5
=item * C
The operator C is not a valid key for C.
However, if the operator C is overloaded then the same
implementation will be used for C
(since the two operators differ only in precedence).
=item * C
The key C is used for unary minus to disambiguate it from
binary C.
=item * C, C
Assuming they are to behave analogously to Perl's C and C,
overloaded implementations of these operators are required to
mutate their operands.
No distinction is made between prefix and postfix forms of the
increment and decrement operators: these differ only in the
point at which Perl calls the associated subroutine when
evaluating an expression.
=item * I
+= -= *= /= %= **= <<= >>= x= .=
&= |= ^= &.= |.= ^.=
Simple assignment is not overloadable (the C key is used
for the L).
Perl does have a way to make assignments to an object do whatever
you want, but this involves using tie(), not overload -
see L and the L examples below.
The subroutine for the assignment variant of an operator is
required only to return the result of the operation.
It is permitted to change the value of its operand
(this is safe because Perl calls the copy constructor first),
but this is optional since Perl assigns the returned value to
the left-hand operand anyway.
An object that overloads an assignment operator does so only in
respect of assignments to that object.
In other words, Perl never calls the corresponding methods with
the third argument (the "swap" argument) set to TRUE.
For example, the operation
$a *= $b
cannot lead to C's implementation of C being called,
even if C is a scalar.
(It can, however, generate a call to C's method for C).
=item * I
+ - * / % ** << >> x .
& | ^ &. |. ^.
As described L,
Perl may call methods for operators like C and C in the course
of implementing missing operations like C, C, and C.
While these methods may detect this usage by testing the definedness
of the third argument, they should in all cases avoid changing their
operands.
This is because Perl does not call the copy constructor before
invoking these methods.
=item * C
Traditionally, the Perl function C rounds to 0
(see L), and so for floating-point-like types one
should follow the same semantic.
=item * I
"" 0+ bool
These conversions are invoked according to context as necessary.
For example, the subroutine for C (stringify) may be used
where the overloaded object is passed as an argument to C,
and that for C where it is tested in the condition of a flow
control statement (like C) or the ternary C operation.
Of course, in contexts like, for example, C, Perl will
invoke C's implementation of C rather than (in this
example) converting C to a number using the numify method
C (an exception to this is when no method has been provided
for C and L is set to TRUE).
The subroutines for C, C, and C can return
any arbitrary Perl value.
If the corresponding operation for this value is overloaded too,
the operation will be called again with this value.
As a special case if the overload returns the object itself then it will
be used directly. An overloaded conversion returning the object is
probably a bug, because you're likely to get something that looks like
C.
qr
The subroutine for C is used wherever the object is
interpolated into or used as a regexp, including when it
appears on the RHS of a C<=~> or C operator.
C must return a compiled regexp, or a ref to a compiled regexp
(such as C returns), and any further overloading on the return
value will be ignored.
=item * I
If CE> is overloaded then the same implementation is used
for both the I syntax C$varE> and
I syntax C${var}E>.
=item * I
The key C is used to specify a subroutine to handle all the
filetest operators (C, C, and so on: see L for
the full list);
it is not possible to overload any filetest operator individually.
To distinguish them, the letter following the '-' is passed as the
second argument (that is, in the slot that for binary operators
is used to pass the second operand).
Calling an overloaded filetest operator does not affect the stat value
associated with the special filehandle C. It still refers to the
result of the last C, C or unoverloaded filetest.
This overload was introduced in Perl 5.12.
=item * I
The key C allows you to override the smart matching logic used by
the C operator and the switch construct (C/C). See
L and L.
Unusually, the overloaded implementation of the smart match operator
does not get full control of the smart match behaviour.
In particular, in the following code:
package Foo;
use overload '~~' => 'match';
my $obj = Foo->new();
$obj ~~ [ 1,2,3 ];
the smart match does I invoke the method call like this:
$obj->match([1,2,3],0);
rather, the smart match distributive rule takes precedence, so $obj is
smart matched against each array element in turn until a match is found,
so you may see between one and three of these calls instead:
$obj->match(1,0);
$obj->match(2,0);
$obj->match(3,0);
Consult the match table in L for
details of when overloading is invoked.
=item * I
${} @{} %{} &{} *{}
If these operators are not explicitly overloaded then they
work in the normal way, yielding the underlying scalar,
array, or whatever stores the object data (or the appropriate
error message if the dereference operator doesn't match it).
Defining a catch-all C (see L)
makes no difference to this as the catch-all function will
not be called to implement a missing dereference operator.
If a dereference operator is overloaded then it must return a
I of the appropriate type (for example, the
subroutine for key C should return a reference to a
scalar, not a scalar), or another object which overloads the
operator: that is, the subroutine only determines what is
dereferenced and the actual dereferencing is left to Perl.
As a special case, if the subroutine returns the object itself
then it will not be called again - avoiding infinite recursion.
=item * I
nomethod fallback =
See L>.
=back
=head2 Magic Autogeneration
If a method for an operation is not found then Perl tries to
autogenerate a substitute implementation from the operations
that have been defined.
Note: the behaviour described in this section can be disabled
by setting C to FALSE (see L).
In the following tables, numbers indicate priority.
For example, the table below states that,
if no implementation for C has been defined then Perl will
implement it using C (that is, by inverting the value
returned by the method for C);
if boolean conversion is also unimplemented then Perl will
use C or, failing that, C.
operator | can be autogenerated from
|
| 0+ "" bool . x
=========|==========================
0+ | 1 2
"" | 1 2
bool | 1 2
int | 1 2 3
! | 2 3 1
qr | 2 1 3
. | 2 1 3
x | 2 1 3
.= | 3 2 4 1
x= | 3 2 4 1
<> | 2 1 3
-X | 2 1 3
Note: The iterator (CE'>) and file test (C)
operators work as normal: if the operand is not a blessed glob or
IO reference then it is converted to a string (using the method
for C, C, or C) to be interpreted as a glob
or filename.
operator | can be autogenerated from
|
| < <=> neg -= -
=========|==========================
neg | 1
-= | 1
-- | 1 2
abs | a1 a2 b1 b2 [*]
< | 1
<= | 1
> | 1
>= | 1
== | 1
!= | 1
* one from [a1, a2] and one from [b1, b2]
Just as numeric comparisons can be autogenerated from the method
for C<< '<=>' >>, string comparisons can be autogenerated from
that for C:
operators | can be autogenerated from
====================|===========================
lt gt le ge eq ne | cmp
Similarly, autogeneration for keys C and C is analogous
to C and C above:
operator | can be autogenerated from
|
| += +
=========|==========================
+= | 1
++ | 1 2
And other assignment variations are analogous to
C and C (and similar to C and C above):
operator || *= /= %= **= <<= >>= &= ^= |= &.= ^.= |.=
-------------------||-------------------------------------------
autogenerated from || * / % ** << >> & ^ | &. ^. |.
Note also that the copy constructor (key C) may be
autogenerated, but only for objects based on scalars.
See L.
=head3 Minimal Set of Overloaded Operations
Since some operations can be automatically generated from others, there is
a minimal set of operations that need to be overloaded in order to have
the complete set of overloaded operations at one's disposal.
Of course, the autogenerated operations may not do exactly what the user
expects. The minimal set is:
+ - * / % ** << >> x
<=> cmp
& | ^ ~ &. |. ^. ~.
atan2 cos sin exp log sqrt int
"" 0+ bool
~~
Of the conversions, only one of string, boolean or numeric is
needed because each can be generated from either of the other two.
=head2 Special Keys for C
=head3 C
The C key is used to specify a catch-all function to
be called for any operator that is not individually overloaded.
The specified function will be passed four parameters.
The first three arguments coincide with those that would have been
passed to the corresponding method if it had been defined.
The fourth argument is the C key for that missing
method. If the experimental "bitwise" feature is enabled (see L),
a fifth TRUE argument is passed to subroutines handling C, C, C and C to indicate that the caller is expecting numeric behaviour.
For example, if C is an object blessed into a package declaring
use overload 'nomethod' => 'catch_all', # ...
then the operation
3 + $a
could (unless a method is specifically declared for the key
C) result in a call
catch_all($a, 3, 1, '+')
See L.
=head3 C
The value assigned to the key C tells Perl how hard
it should try to find an alternative way to implement a missing
operator.
=over
=item * defined, but FALSE
use overload "fallback" => 0, # ... ;
This disables L.
=item * C
In the default case where no value is explicitly assigned to
C, magic autogeneration is enabled.
=item * TRUE
The same as for C, but if a missing operator cannot be
autogenerated then, instead of issuing an error message, Perl
is allowed to revert to what it would have done for that
operator if there had been no C directive.
Note: in most cases, particularly the L,
this is unlikely to be appropriate behaviour.
=back
See L.
=head3 Copy Constructor
As mentioned L,
this operation is called when a mutator is applied to a reference
that shares its object with some other reference.
For example, if C is mathemagical, and C is overloaded
with C, and C is overloaded with C, then the
code
$a = $b;
# ... (other code which does not modify $a or $b) ...
++$b;
would be executed in a manner equivalent to
$a = $b;
# ...
$b = $b->clone(undef, "");
$b->incr(undef, "");
Note:
=over
=item *
The subroutine for C does not overload the Perl assignment
operator: it is used only to allow mutators to work as described
here. (See L above.)
=item *
As for other operations, the subroutine implementing '=' is passed
three arguments, though the last two are always C and C.
=item *
The copy constructor is called only before a call to a function
declared to implement a mutator, for example, if C in the
code above is effected via a method declared for key C
(or 'nomethod', passed C as the fourth argument) or, by
autogeneration, C.
It is not called if the increment operation is effected by a call
to the method for C since, in the equivalent code,
$a = $b;
$b = $b + 1;
the data referred to by C is unchanged by the assignment to
C of a reference to new object data.
=item *
The copy constructor is not called if Perl determines that it is
unnecessary because there is no other reference to the data being
modified.
=item *
If C is undefined or TRUE then a copy constructor
can be autogenerated, but only for objects based on scalars.
In other cases it needs to be defined explicitly.
Where an object's data is stored as, for example, an array of
scalars, the following might be appropriate:
use overload '=' => sub { bless [ @{$_[0]} ] }, # ...
=item *
If C is TRUE and no copy constructor is defined then,
for objects not based on scalars, Perl may silently fall back on
simple assignment - that is, assignment of the object reference.
In effect, this disables the copy constructor mechanism since
no new copy of the object data is created.
This is almost certainly not what you want.
(It is, however, consistent: for example, Perl's fallback for the
C operator is to increment the reference itself.)
=back
=head2 How Perl Chooses an Operator Implementation
Which is checked first, C or C?
If the two operands of an operator are of different types and
both overload the operator, which implementation is used?
The following are the precedence rules:
=over
=item 1.
If the first operand has declared a subroutine to overload the
operator then use that implementation.
=item 2.
Otherwise, if fallback is TRUE or undefined for the
first operand then see if the
L
allows another of its operators to be used instead.
=item 3.
Unless the operator is an assignment (C, C, etc.),
repeat step (1) in respect of the second operand.
=item 4.
Repeat Step (2) in respect of the second operand.
=item 5.
If the first operand has a "nomethod" method then use that.
=item 6.
If the second operand has a "nomethod" method then use that.
=item 7.
If C is TRUE for both operands
then perform the usual operation for the operator,
treating the operands as numbers, strings, or booleans
as appropriate for the operator (see note).
=item 8.
Nothing worked - die.
=back
Where there is only one operand (or only one operand with
overloading) the checks in respect of the other operand above are
skipped.
There are exceptions to the above rules for dereference operations
(which, if Step 1 fails, always fall back to the normal, built-in
implementations - see Dereferencing), and for C (which has its
own set of rules - see C under L
above).
Note on Step 7: some operators have a different semantic depending
on the type of their operands.
As there is no way to instruct Perl to treat the operands as, e.g.,
numbers instead of strings, the result here may not be what you
expect.
See L.
=head2 Losing Overloading
The restriction for the comparison operation is that even if, for example,
C should return a blessed reference, the autogenerated C
function will produce only a standard logical value based on the
numerical value of the result of C. In particular, a working
numeric conversion is needed in this case (possibly expressed in terms of
other conversions).
Similarly, C and C operators lose their mathemagical properties
if the string conversion substitution is applied.
When you chop() a mathemagical object it is promoted to a string and its
mathemagical properties are lost. The same can happen with other
operations as well.
=head2 Inheritance and Overloading
Overloading respects inheritance via the @ISA hierarchy.
Inheritance interacts with overloading in two ways.
=over
=item Method names in the C directive
If C in
use overload key => value;
is a string, it is interpreted as a method name - which may
(in the usual way) be inherited from another class.
=item Overloading of an operation is inherited by derived classes
Any class derived from an overloaded class is also overloaded
and inherits its operator implementations.
If the same operator is overloaded in more than one ancestor
then the implementation is determined by the usual inheritance
rules.
For example, if C inherits from C and C (in that order),
C overloads C with C, and C overloads
C by C, then the subroutine C will
be called to implement operation C for an object in package C.
=back
Note that in Perl version prior to 5.18 inheritance of the C key
was not governed by the above rules. The value of C in the first
overloaded ancestor was used. This was fixed in 5.18 to follow the usual
rules of inheritance.
=head2 Run-time Overloading
Since all C directives are executed at compile-time, the only way to
change overloading during run-time is to
eval 'use overload "+" => \&addmethod';
You can also use
eval 'no overload "+", "--", "<="';
though the use of these constructs during run-time is questionable.
=head2 Public Functions
Package C provides the following public functions:
=over 5
=item overload::StrVal(arg)
Gives the string value of C as in the
absence of stringify overloading. If you
are using this to get the address of a reference (useful for checking if two
references point to the same thing) then you may be better off using
C, which is faster.
=item overload::Overloaded(arg)
Returns true if C is subject to overloading of some operations.
=item overload::Method(obj,op)
Returns C or a reference to the method that implements C.
=back
=head2 Overloading Constants
For some applications, the Perl parser mangles constants too much.
It is possible to hook into this process via C
and C functions.
These functions take a hash as an argument. The recognized keys of this hash
are:
=over 8
=item integer
to overload integer constants,
=item float
to overload floating point constants,
=item binary
to overload octal and hexadecimal constants,
=item q
to overload C-quoted strings, constant pieces of C- and C-quoted
strings and here-documents,
=item qr
to overload constant pieces of regular expressions.
=back
The corresponding values are references to functions which take three arguments:
the first one is the I string form of the constant, the second one
is how Perl interprets this constant, the third one is how the constant is used.
Note that the initial string form does not
contain string delimiters, and has backslashes in backslash-delimiter
combinations stripped (thus the value of delimiter is not relevant for
processing of this string). The return value of this function is how this
constant is going to be interpreted by Perl. The third argument is undefined
unless for overloaded C- and C- constants, it is C in single-quote
context (comes from strings, regular expressions, and single-quote HERE
documents), it is C